1. Field of the Invention:
This invention relates to front conversion lenses attachable to the front of a photographic lens to change the focal length of the entire photographic system, and more particularly to conversion lenses of reduced weight in the entire lens system with a high optical performance suited for use in photographic cameras or video cameras.
2. Description of the Related Art:
There have been proposed front conversion lenses which, when attached to the photographic lens at the front, extend the focal length of the entire photographic system while maintaining constant the position of the focal plane of the entire system, for example, in Japanese Laid-Open patent application Nos. SHO 55-32046 and SHO 59-204817.
The telephoto conversion lens has, in most cases, two lens units of which the front is positive in refractive power and the rear is negative, and the wide-angle conversion lens has, in most cases, two lens units of which the front is negative in refractive power and the rear is positive. The entirety of these two lens units constitutes an afocal system. Therefore, applicant uses the term unit as equivalent to the term group, since the art recognizes that both the terms unit and group may mean one or more optical elements, having an identified optical function. In the simplest form, therefore, the lens system can be constructed by the combination of two lenses of positive and negative powers.
However, the conversion lens for attachment to the front of the photographic lens tends to increase the size of the entire lens system as compared with the rear conversion lens, and it tends to be very heavy. It is, therefore, very difficult to achieve a minimization of the size and weight of the whole of the conversion lens in such a manner as to preserve good optical performance.
A good compromise between the requirements of minimizing the diameter of the front member of the conversion lens and of stabilizing good correction of chromatic aberrations, particularly lateral chromatic aberration, is generally obtained when the front and rear lens units are constructed and arranged in such a way that the air separation therebetween is made so short, preferably, as if it were filled with glass material.
On this account, a lens design that most of the conventional high performance conversion lenses have employed is such that a plurality of lenses are used in constructing each of the front and rear lens units with the result that the air separation between both units becomes short.
Hence, the use of the prior known lens design for improving the optical performance of the conversion lens has resulted in a great increase of the weight with a great inconvenience to handling it.
In order to reduce the weight of the whole of the conversion lens, all the lens elements of the conversion lens may be made of plastic materials. This idea has found its use in some of the commerically available items. However, because plastic has weak physical strengths, and since as with the conversion lens, the attaching and detaching is frequently repeated, the possibility of damaging the lens system particularly at the exposed lens surface to the atmosphere is very high, constituting a cause of lowering the optical performance.